1. Field of the Invention
This invention relates generally to filterless amplifiers, and more particularly, to a filterless modulation scheme to reduce the total harmonic distortion (THD) and noise (N) associated with filterless amplifiers below that achievable using traditional modulation schemes.
2. Description of the Prior Art
Class-D amplifiers are two to five time more efficient than class-AB amplifiers. Because of their greater efficiency, class-D amplifiers require smaller power supplies and eliminate heat sinks, significantly reducing overall system costs, size and weight. Class-D amplifiers are not in many portable products today because the traditional class-D amplifier requires and output filter, which increases size by approximately 75% and solution cost by approximately 30%. Filterless class-D amplifiers eliminate the output filter while keeping the efficiency benefit. The filterless modulation scheme brings class-D amplifiers approximately equal to class-AB amplifiers in cost and size, by has the great efficiency advantages. The THD+N of the filterless modulation scheme is better than the THD+N of equivalent class-D amplifiers with the traditional modulation scheme.
A method to achieve filterless class-D operation, increased efficiency, and reduced cost is to deliver current to the load only when needed, and once delivered, maintain the current (try not to decay or waste energy in removing the current). A quaternary modulation scheme is implemented in an H-bridge configuration. FIG. 1 is a block diagram implementation of a quaternary modulation scheme that is familiar to those skilled in the art. The quaternary modulation scheme shown in FIG. 1 has four states of operation as shown in FIG. 2, and voltage and current waveforms as shown in FIG. 3. The modulation scheme uses any of the four states in any order, depending on the audio input signal.
When a positive audio signal is present, waveforms change to that shown in FIG. 3, wherein the edges of OUTP move away from each other, and the edges of OUTN move towards each other. The resulting differential signal across the bridge-tied-load consists of narrow pulses with polarity determined by the polarity of the audio input signal. These narrow pulses also have a secondary desired affect of doubling the differential pulse width modulation (PWM) frequency. This then achieves the desired effect of placing current into the load only when needed, resulting in increased efficiency and less energy loss in the load (speaker).
Eliminating the filter also causes the amplifier to radiate electromagnetic interference (EMI) from the wires connecting the amplifier to the speaker. EMI is defined as the radiation or transfer of electromagnetic energy through a non-metallic medium, such as air, that interferes with the operation of other electronic devices in the vicinity. An instantaneous change in current results in a magnetic (H) field, while an instantaneous change in a voltage results in an electric (E) field, both of which may cause EMI. The electric field, which is a common-mode effect, could in fact be quite large from the switching voltage if the speaker wires are long enough. A ferrite bead filter can be added to reduce the EMI radiation at 10 MHz and higher. The cut-off frequency, however, is limited because the inductance of the ferrite bead is limited due to size constraints and the capacitor is limited because the added capacitance increases the supply current and thus decreases the efficiency. The rail-to-rail common-mode voltage at 50% duty cycle causes an increase in supply current if the output of the amplifiers see too high of a capacitance.
In view of the foregoing, a need exists for a modulation scheme for filterless switching amplifiers with reduced EMI.
The present invention is directed to a ternary modulation scheme for filterless switching amplifiers with reduced EMI. The ternary modulation scheme reduces the common mode component of the signal via a logic block added between the output of comparators and the H-bridge described herein before with reference to FIG. 1. The logic block reduces the common mode component of the signal by allowing only one of the states with zero differential voltage across the load to exist. State 3 shown in FIG. 2, for example, is completely eliminated. Eliminating the common mode signal significantly reduces EMI since it reduces the electric field (E) which is responsible for EMI. The ternary modulation scheme thus reduces the possible number of states at the outputs to allow filterless operation while eliminating the switching of the common mode voltage at the modulation frequency.
In one aspect of the present invention, a ternary modulation scheme is provided to allow implementation of a class-D amplifier without the need for an output filter to achieve lower EMI than that achievable using the quaternary modulation scheme.
In another aspect of the present invention, a ternary modulation scheme is provided to reduce the possible number of states at the outputs and to eliminate switching of the common mode voltage at the modulation frequency.
In still another aspect of the present invention, a ternary modulation scheme is provided to reduce the cut-off frequency of a ferrite bead filter by reducing the size of the ferrite bead and increasing capacitance of the capacitor without increasing amplifier size or cost.
In yet another aspect of the present invention, a ternary modulation scheme is provided to reduce quiescent current when using an LC filter.
In still another aspect of the present invention, a ternary modulation scheme is provided to allow use of a smaller and less expensive LC filter without increasing the quiescent current or output ripple voltage for amplifiers that employ an LC filter.
According to one embodiment of the invention, a switching amplifier modulation circuit comprises:
a switching amplifier having a differential output; and a logic circuit operational to allow only one signal state with zero differential voltage across a load connected to the differential output to exist such that the common mode component of a differential signal across the load is substantially reduced below that achievable using modulation schemes that allow more than one signal state with zero differential voltage to exist across the load.
According to another embodiment of the invention, a switching amplifier modulation circuit comprises a logic circuit responsive to switching amplifier positive and negative logic signals generated in response to an audio input signal such that the logic circuit is operational to allow only one signal state with zero differential voltage across a load connected to a differential output associated with a switching amplifier to exist such that the common mode component of a differential signal across the load is substantially reduced below that achievable using modulation circuits that allow more than one signal state with zero differential voltage to exist across the load.
According to yet another embodiment of the invention, a switching amplifier modulation circuit comprises a switching amplifier having a differential output; and signal generating means for generating no more than one signal state with zero differential voltage across a load connected to the differential output to exist such that the common mode component of a differential signal across the load is substantially reduced below that achievable using modulation circuits that allow more than one signal state with zero differential voltage to exist across the load.
According to still another embodiment of the present invention, a switching amplifier modulation circuit comprises a differential amplifier responsive to an audio input signal to generate a differential amplifier output signal; a positive integrator responsive to the differential amplifier output signal to generate a positive integrator signal; a negative integrator responsive to the differential amplifier output signal to generate a negative integrator signal; a ramp generator operational to generate a ramp signal; a first comparator responsive to the ramp signal and the positive integrator signal to generate a first logic signal; a second comparator responsive to the ramp signal and the negative integrator signal to generate a second logic signal; an H-bridge circuit having a switching amplifier differential output; and a logic circuit responsive to the first and second logic signals to drive the H-bridge circuit such that the H-bridge circuit can generate a differential signal across a load connected to the switching amplifier differential output and wherein the logic circuit is operational to allow only one signal state with zero differential voltage across the load to exist such that the common mode component of the differential signal across the load is substantially reduced below that achievable using modulation schemes that allow more than one signal state with zero differential voltage to exist across the load.